The present invention relates to micro-mechanical light modulators and to Spatial Light Modilators (SLMs) including arrays of such modulators.
Various optical applications, such as projection, imaging and optical fiber communication, require light modulation and/or light beam steeped In optical applications where a plurality of optical beams should be handled simultaneously, the modulation can be achieved by using optical modulators called Spatial Light Modulators (SLMs) or Light Valves (LVs), which are arrays of individually controlled members. Distinctive class SLMs work in diffractive mode; An activated individual member of the SLM array diffracts the incoming light beam at a discrete multitude of angles, these angels being a function of the light wavelength and the dimensions of the modulator. Such modulators, based on Micro Elctro-Mechanical Systems (MEMS) technology and called Deformable Diffractive Gratings, are described, for example, in U.S. Pat. Nos. 5,311,360; 5,459,610 to The Board of Trustees of the Leland Stanford, Junior University; U.S. Pat. Nos. 5,629,801; 5,661,592 to Silicon Light Machines; U.S. Pat. No. 5,677,783 to The Board of Trams of the Leland Stanford, Junior University; U.S. Pat. Nos. 5,808,797; 5,841,579; 5,982,553 to Silicon Light Machines; U.S. Pat. No. 5,920,518 to Micron Technology, Inc.; U.S. Pat. No. 5,949,570 to Matsushita Electric Industrial Co.; U.S. Pat. No. 5,999,319 to InterScience Inc.; U.S. Pat. Nos. 6,014,257; 6,031,652 to Eastman Kodak Company.
In the conventional art Deformable Diffractive Gratings light modulation systems, the diffractive element is usually of xe2x80x9cpistonxe2x80x9d type or cantilever mirror type. Both types of diffractive elements have some advantages, while suffering from some drawbacks. For example, a piston diffractive grating element is always faster than a cantilever mirror diffractive grating element, however, its efficiency is lower. Reference is made now to FIGS. 1, 2a, 2b and 2c, which show a typical conventional art design of a piston diffractive type element and demonstrate its operation. Throughout the figures, similar elements are noted with similar numeral references.
FIG. 1 is a schematic isometric view of a conventional art piston type deformable grating element 10. The element 10 consists of several beams, noted 25, created by a photolithographic process in a frame 20. The beams 25 define a diffractive grating 22, supported by the etched structure 30. The bee 25 rest on a silicon substrate base 40. Beams 21 of the beams 25 are movable and are suspended over gaps 41, which are etched in the silicon substrate base 40, while other beams 23 of the beams 25 are static. The beams 25 are coated with a reflective layer 60. This reflective layer 60 is conductive and functions as an electrode. An opposite electrode 50 is deposited on the opposite side of the silicon substrate 40.
FIGS. 2a and 2b show the Axe2x80x94A cross-section of the conventional art modulator 10 of FIG. 1 in non-active and active states, respectively. In FIG. 2a, no voltage is applied between the suspended beams 21 and the common electrode 50. Accordingly, all the beams 21 and 23 are coplanar and the diffractive element works as a plane mirror, i.e. incident beam 70 and reflected beam 71 are in the exact opposite directions. When voltage is applied between the suspended beams 21 and the common electrode 50, as shown in FIG. 2b, the suspended beams 21 are deformed in the direction of the electrical field created by the applied voltage. Thus, the non-suspended beams 23 and the suspended beams 21 define a diffractive structure returing an incident beam 70 in directions 171. The directions 171 and the direction 70 of the incident beam constitute an angle "PHgr" which follows the laws of diffractive optics and is called a diffractive angle. The angle "PHgr" is a function of the light wavelength xcex and the grating period d. The diffraction efficiency is a function of the grating amplitude. For piston type grating, the optimal amplitude for achieving optimal efficiency, is xcex/4, as illustrated in FIG. 2b. In this example and the example below it is assumed that the light modulation system operates in air with refractive index n=1.
FIG. 2c shows the angular distribution of the light energy for non-active (thin line) and active (thick line) xcex/4 optimize piston type deformable grating light modulating element. The calculations are made for Fraunhofer diffraction of parallel light beam while xcex=830 nm and grating period d=10 xcexcm, and while King into account the interference of two simultaneously working elements (i.e. 2d xe2x80x98UPxe2x80x99-xe2x80x98DOWNxe2x80x99-xe2x80x98UPxe2x80x99-xe2x80x98DOWNxe2x80x99 structure). It can be seen from this figure that when the element is active, most of the energy is distributed in the +1st and xe2x88x921st orders, while when it is non-active, most of the energy is distributed in the xe2x80x9czeroxe2x80x9d order (tinner line).
Commonly, there are two kinds of distinctive optical systems that utilize diffractive type light modulators: optical light systems having spatial filtering of the xe2x80x9czeroxe2x80x9d order, and optical light systems having spatial filtering of the xc2x11st and higher orders. When the xe2x80x9czeroxe2x80x9d order is filtered, the maximal theoretical energy efficiency is 70%, while when the xc2x11st and higher orders are filtered, the maximal theoretical energy efficiency can be as high as 90%. In both cases, the maximal theoretical contrast ratio (the ratio between the energies passing the spatial filter in the active and non-active states, respectively) that can be achieved is 1:12.
However, for most applications, such as pre-press imaging and projection displays, contrast ratio as low as 1:12 is unacceptable. An additional disadvantage of the piston type diffractive grating modulators, is that when in active state, the light energy is distributed symmetrically in the xc2x11st and higher orders, which in many cases can lead to a more complex optical system, as the light has to be cutoff from both sides of the maximum.
There is provided in accordance with an embodiment of the invention, a light valve of deformable grating type. The light valve includes at least three beams, one beam of being of a substantially fixed-position, and at least two beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam, and first and second electrodes for transmitting electrostatic force to at least the deformable beams.
There is also provided in accordance with a further embodiment of the invention, a light valve of deformable grating type, which includes at least three beams, one beam being of a substantially fixed-position, and the three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam and a first electrode and a second electrode, the electrodes transmitting electrostatic force to the deformable beams.
In addition, there is also provided in accordance with an embodiment of the invention, a method for light modulation. The method includes the steps of:
providing a light valve of deformable grating type, the light valve includes at least three beams, at least the first beam of the at least three beams being of a substantially fixed-position, and at least two beams of the at least three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam;
illuminating the light valve,; and
applying voltage between the first electrode and the second electrode.
providing a light valve of deformable grating type, the light valve includes at least three beams, at least the first beam of the at least three beams being of a substantially fixed-position, and the at least three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam;
illuminating the light valve; and
applying voltage between the first electrode and the second electrode.
Furthermore, in accordance with an embodiment of the invention, the deformable beams form the first electrode and the second electrode is common to all the deformable beams.
Furthermore, in accordance with an embodiment of the invention, the deformable beams form the first electrode and the second electrode includes an array of electrodes, each electrode of the array of electrodes associated with one of the deformable beams.
Furthermore, in accordance with an embodiment of the invention, the first electrode includes an array of electrodes, each electrode of the array of electrodes associated with one of the deformable beams, and the second electrode is common to all the deformable beams.
In addition, in accordance with an embodiment of the invention, a spatial light modulator is formed as an array of light valves.
Furthermore, in accordance with an embodiment of the invention, the beam of a substantially fixed-position is deformable by electrostatic force.
Furthermore, in accordance with an embodiment of the invention, the at least three beams form the first electrode and the second electrode is common to all the deformable beams. Alternatively, the at least three beams form the first electrode and the second electrode includes an array of electrodes, each electrode of the array of electrodes associated with one of the at least three beams.
Furthermore, in accordance with an embodiment of the invention, the first electrode includes an array of electrodes, each electrode of the array of electrodes associated with one of the at least three beams, and the second electrode is common to all the at least three beams.